1. Field of the Invention
This invention relates generally to the field of precisely aligning photoresist masks to both the front and back sides of a silicon wafer and, more particularly, to a fixture-less process employing a laser beam for scribing and alignment mark on the back side of a wafer having an heavily doped interior layer in which microscopic defects are induced by the laser beam and, then, heating the wafer to cause the defects to propagate to the front side of a wafer where they appear as an identically positioned image of the alignment mark scribed on the back side.
2. Description of the Prior Art
The problem of aligning both the back and front sides of a semiconductor wafer to a photographic mask has been recognized in the prior art. U.S. Pat. No. 3,752,589--Kobayashi discloses an optical-mechanical means and method for aligning the pattern of a photographic mask on one side of a wafer to a pattern on the opposite side, wherein individual optical images of the patterns on both sides of the wafer are obtained and, then, the position of the mask is mechanically adjusted until the two optical images are aligned.
U.S. Pat. No. 4,046,985--Gates discloses an optical-mechanical semiconductor wafer alignment apparatus for repeatedly aligning the back sides of wafers to the axis of a kerf-cutting laser. First, a transparent mask, having on its front side a duplicate of the device pattern and of the alignment marks on the opaque wafers, is visually aligned to the laser, and the apparatus is set to maintain this alignment. Thus, subsequently inserted opaque semiconductor wafers, having the same alignment marks, will travel through the apparatus along the same path as the transparent mask, including inversion of the wafers, so that their back sides are aligned to the laser whose beam automatically impinges on the kerf areas between the devices.
U.S. Pat. No. 4,318,752--Tien discloses a method of forming a p-doped area in an n-doped layer of a plural layer semiconductor body by passing a laser beam through a first p-doped layer into an adjacent n-layer which absorbs the laser beam energy and generates heat to cause diffusion of the p-doped layer into the n-doped layer. The wavelength of the laser beam is chosen such that is passes through the p-doped layer without absorption.
U.S. Pat. No. 4,131,487--Pearce et al discloses a process for gettering a semiconductor wafer with a laser beam, wherein a laser beam is directed upon the back side of the wafer to create lattice damage and strain in the back side. Subsequent heating of the wafer releases the strain to attract mobile defects from the front side of the wafer to the back side.